Spacecraft such as probes and satellites generally support devices having energy detecting capabilities of one type or another. Remote sensing devices such as infrared, optical, and radio telescopes are examples of such detecting devices. Typical remote sensing devices provide measurement of reflected (primarily solar) or emitted (from man-made sources) visible and near-infrared energy from the Earth or other heavenly bodies. A method of calibrating the measured radiance from Earth (or other source) is to create a reference radiance using a ubiquitous, known source of spectral irradiance, such as the Sun, as reference input to a diffusive reflector which in turn provides a known radiance to the remote sensing instrument aperture.
The standard radiance value may be created by reflecting known solar spectral irradiance from the diffuser panel toward the remote sensing device during an occasional (non-normal operation) calibration. The remote sensing device output is measured as the device receives the known diffusely reflected energy from the diffuser panel. Using the linear characteristics of the remote sensing device and a reference view of empty space (i.e., no significant irradiance at the remote sensing device aperture), response of the device to the known radiance input is determined. This radiance calibration process provides sufficient information to calculate radiance incident at the device aperture during normal operation using the instrument output as it views the Earth or other target of interest.
The spectral reflectance characteristics of the diffuser panel, however, may change with time due to degradation of the diffuser panel. Since the diffuser panel is employed as the reference source, any change, i.e., degradation of the diffusive surface material, results in a distortion in the measurements of the remote sensing device.
According to U.S. Pat. No. 5,716,030, an attempt is made to prevent the diffuser surface from degrading by providing a door having a first calibration surface disposed on a first panel to provide a radiant temperature reference for infrared calibrations. A second calibration surface is disposed on a second panel to provide an absolute radiance reference for optical calibrations. A hinge controls the relative positioning of the calibration and reference panels with respect to each other and with respect to the remote sensor.
Another method of accounting for diffuser surface degradation, according to U.S. Pat. No. 6,597,457, is through a device that calibrates the diffuser panel by sighting the sun via a port, and detecting the intensity of radiation within an averaging chamber in which a remote sensor is disposed. The radiation averaging chamber is constructed in a spherical shape having a diffuse inner reflecting surface to induce multiple reflection of radiation to accomplish an averaging of the radiation. Two ports are provided in a wall of the chamber for entry of radiation, a first port is employed for viewing solar radiation reflected by the diffuser panel, and a second port is employed for sighting solar radiation propagating directly from the sun to the second port.